This invention relates to the combustion of coal and in particular to the generation of sorbents to capture mercury (Hg) in flue gas generated during coal combustion.
Emissions from coal combustion may contain volatile metals such as mercury (Hg). There is a long felt need to reduce Hg in gaseous emissions from coal-fired boilers and other industrial coal combustion systems. As mercury volatizes during coal combustion, it enters the flue gas generated by combustion. Some of the volatized mercury can be captured by injected sorbents and removed via a particulate collection system. If not captured, the mercury may pass into the atmosphere with the stack gases from the coil boiler. Mercury is a pollutant. Accordingly, it is desirable to capture a much mercury in flue gas before the stack discharge.
Injection of activated carbon as a sorbent that captures mercury in the flue gas is a known technology for Hg control. See e.g., Pavish et al., “Status review of mercury control options for coal-fired power plants” Fuel Processing Technology 82, pp. 89-165 (2003). Depending on coal type and the specific configuration of the emission control system, e.g., injection ahead of a particulate collector or a compact baghouse added behind an existing electrostatic particulate control device ESP, and coal type, the efficiency of Hg removal by activated carbon injection ranges from 60% to 90%.
The cost of Hg control in coal-fired power plants using activated carbon tends to be expensive. See e.g., Brown et al., “Control of Mercury Emissions from Coal-Fired Power Plants: A Preliminary Cost Assessment and the Next Steps for Accurately Assessing Control Costs”, Fuel Processing Technology 65-66, pp. 311-341 (2000). The typical cost for mercury removal using activated carbon injection generally ranges $20,000 per pound (lb.) of removed mercury to $70,000/lb of Hg. This cost is dominated by the cost of the sorbent. Accordingly there is a long felt need for an economical way to produce activated carbon sorbents. By reducing the cost of sorbents, the cost of removing mercury from flue gas may be substantially reduced.